Can I do the following (seems simpler to me, no need to mount anything):
1. Check that my current back-up drive is up-to-date and capable of booting the computer
2. Replace the old internal iMac drive with new
3. Format the new drive (any need to partition Mac?)
3. Use SuperDuper to clone the back-up drive to the new
I would use something like a docking station
1. Connect new drive to adapter (see above), format.
2. Clone boot drive to new drive with SuperDuper, Disk Utility or CarbonCopy Cloner.
3. Test new drive (still externally connected) thoroughly (boot, launch apps, etc)
4. Install new drive in your iMac.
I agree with John and Andrew, I never saw any speed benefit in partitioning the boot drive.